|HAS FULL AZIMUTH IMAGING GONE FULL CIRCLE?|
During the summer of 2005 the EAGE and SEG organised a Summer Research Workshop, dedicated to multi-component seismic, and held in Pau, France. During this workshop it was observed that the largest increase, from the previous Boise (2000) workshop, in “proven” multi-component capability was in reservoir monitoring. Surprisingly, the driver behind the multi-component business was not shear waves but better pressure wave data (Lynn and Spitz, 2006). Amongst the benefits cited was wider azimuth illumination, though this benefit has been recognised for some time in land seismic (Cordsen and Galbraith, 2002). More recently these benefits have been recognised in the special sessions dedicated the marine multi-azimuth and wide-azimuth seismic held at the SEG Annual Meeting in 2006 and the Offshore Technology Conference in 2007. This interest was documented in the Leading Edge in special section in (April 2007).
On the Norwegian continental shelf the story for full azimuth imaging gained momentum when in 1997, Statoil acquired a 3D OBC dataset at Statfjord, which led to significant improvements in fault definition and a better resolution of small scale structures (Rognø et al, 1999, Osmundsen et al, 2002). This was mainly attributed to the full azimuth and rich offset distribution, which result in better illumination (Thompson et al, 2002). In addition, combining geophone and hydrophone measurements led to a better suppression of water-layer related multiples. Further studies into the role of azimuth using advanced depth imaging techniques (Arntsen and Thompson, 2003), whereby both conventional 3D marine seismic and 3D OBS data were compared, corroborated the importance of azimuth and fold after the survey was acquired. Through extensive practical experience with the Statfjord survey, Statoil soon fully realized and took advantage of the full azimuth acquisition solution offered by 3D OBS. Since 1997, OBS surveys have been carried out over a large number of Statoil’s geologically complex North Sea oil and gas fields, providing detailed, structural images of the disposition of fault-bounded compartments. More recently Statoil carried out an intensive modelling study of the Heidrun field, in collaboration with WesternGeco, to understand the requirements for full azimuth imaging utilising conventional streamer vessels. This culminated in a COIL field trial in 2007. A review of past, present and future trends for full azimuth imaging in the North Sea will be presented.
The 3D OBS survey was acquired around the B platform, where the main objective of the survey was to improve the seismic imaging of the structurally complex East Flank. The quality of the conventional seismic images was affected by gas in the overburden and multiples in the lower reservoir zones. Once the 3D OBS survey was acquired and analysed it was possible to see that the definition of the Base Cretaceous unconformity and the Base Slope of Failure had improved over a large portion of the survey area (Figure 1). More accurate definition of faults and improved resolution of small scale structural elements were also achieved. The new interpretation resulted in more confident mapping of intact rotated fault blocks with better understanding of the areal extension and the internal stratigraphic dip within the East Flank area, (Osmundsen et al., 2002).
Figure 1: (a) 3D OBS image of the Statfjord field showed improved definition relative to the (b) conventional 3D marine seismic image.
It was through the original Statfjord 3D OBS that the imaging benefits of this acquisition were observed.
|THE CIRCLE REACHES THE GULF OF MEXICO|
This early work using OBC on Statfjord for imaging and consequent efforts to study acquisition geometry and data quality Thompson, 1999 was later recognised (Figure 2), Bouska J., 2008 and successfully applied in other regional settings. These efforts in the North Sea went on to have an impact in the Gulf of Mexico where Wide Azimuth Imaging is now recognised as a key factor subsalt imaging, though implemented using multiple conventional source and stream vessels.
Figure 2: An early study of acquisition geometry versus quality presented at FORCE seminar in 1999, and later published by Bouska, 2008
|THE CIRCLE REACHES HEIDRUN|
More recently efforts have been made to transfer the experience of using multiple source and streamer vessels back to the North Sea. In this case a modelling study was initiated on the Heidrun field to evaluate, which conventional streamer acquisition methodologies could provide similar uplift in imaging as experienced from OBC.
|MODELLING AND FIELD TRIAL|
In the Heidrun modelling study, images obtained from wide and full azimuth survey designs, the latter including a coil shooting geometry, were compared with images from a conventional narrow azimuth design. The study showed that a design with increased crossline offset and fold generally leads to fewer artifacts and better noise suppression (Houbiers et al., 2008). Amplitudes along horizons are more consistent, and some flanks which are invisible in the narrow azimuth design, are properly imaged in the full and wide azimuth designs. Moreover, the acquisition footprint is less with the coil design.
To validate the conclusions from the modeling study, the research project was extended with a small field trial with coil shooting design at Heidrun. The target area of the coil shooting field trial is a square area of 2.625 x 2.625 km. The survey design consisted of a ‘dahlia’ pattern with 20 intersecting coils covering the target area. Due to the presence of surface obstructions (the Heidrun tension leg platform and two loading buoys), two coils were removed, and the remaining coils were modified slightly, such that the dahlia pattern became slightly irregular. A straight infill line was acquired to compensate for missing azimuths.
The data were acquired during a four-day-period in September 2008, using a conventional seismic vessel with a single source, towing 10 streamers with length of 4500 m, streamer separation of 75 m, and group distance of 12.5 m. The sailing coils had an approximate radius of 5625 m, and source separation was 25 m. The fold of the survey, color coded in 25 x 25 m bins, is displayed in Figure 1. The small square in the middle denotes the target area. Note that the target area has a full azimuth coverage with high fold, except for the lower right corner of the target zone.
Figures 4 and 5 show images from the conventional and the coil shooting survey (overlain the image from the conventional survey) along a cross-line and an inline, respectively, close to the middle of the target zone. One can clearly see differences when comparing the images from the full and narrow azimuth survey designs.
Figure 4 Images along a cross-line close to the middle of the target zone from A) the conventional survey, and B) the coil shooting survey. The letters indicate the BCU flank (B), a coal marker below the reservoir section (C), and a dip conflict (D).
The flank of the Base Cretaceous Unconformity (BCU) is imaged well in the coil shooting data, see Figure 4. Moreover, one can see differences in the internal pattern above and below the BCU (a more transparent limestone versus a stripy Jurassic section with sub-parallel reflectors), and different reservoir sections are fairly well resolved. In the conventional image, the internal pattern is stripy both below and above the BCU. This indicates a fair attenuation of noise and multiples from the coil survey data.
Figure 5 Images along an inline close to the middle of the target zone from A) the conventional survey and B) the coil shooting survey. The letters in image A) indicate remnant noise (N), migration artifacts/remnant multiple energy (M), and dip conflict (D). The letters in image B) indicate subparallel reflections (P).
Over the last decade we have experienced several step-changes in marine seismic acquisition technologies. Because acquisition and seismic imaging are inextricably linked, these changes have impacted positively on the seismic image of subsurface geology. It has been demonstrated that imaging improvements have been achieved by utilising ocean bottom seismic. This in turn has influenced acquisition and imaging techniques in the Gulf of Mexico, through the introduction of wide azimuth acquisition (WAZ). These developments have then led to new full azimuth technique (FAZ), utilising COIL techniques, which have been piloted at the Heidrun field. These latest technques show that full azimuth has in fact gone full circle.
The authors would like to thank Statoil for permission to publish the paper.
Lynn H., Spitz S.,  Report on the Summer 2005 EAGE-SEG workshop on multi-component seismic, First Break, 24, 7-11.
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Osmundsen, I.K., E. Magerøy, and R. Sørbel, 2002, Multi-Azimuth PZ data, a step forward in fault definition on Statfjord: Presented at the FORCE Seminar “4C/OBS Data Processing and Interpretation - Old Myths, New Insight”, www.force.org.
Thompson, M., 1999, Statfjord 3D/4C data - Azimuth Processing. "Alice through the looking glass", www.force.org
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Houbiers, M., Arntsen, B., Thompson, M., Hager, E., Brown, G., and Hill, D.  Full azimuth seismic modelling at Heidrun. PETEX Conference, London, UK.